The widespread and increasing interest in enhancing biosensing technologies by increasing their sensitivities and lowering their costs has led to the exploration and application of complex nanomaterials as signal transducers and enhancers. In this work, the electrochemical properties of monodispersed AuAg alloy nanoshells (NSs) with finely tunable morphology, composition, and size are studied to assess their potential as electroactive labels. The controlled corrosion of their silver content, caused by the oxidizing character of dissolved oxygen and chlorides of the electrolyte, allows the generation of a reproducible electrochemical signal that is easily measurable through voltammetric techniques. Remarkably, the underpotential deposition of dissolved Ag catalyzed on AuAg NS surfaces is observed and its dependence on the nanoparticle morphology, size, and elemental composition is studied, revealing a strong correlation between the relative amounts of the two metals. The highest catalytic activity is found at Au/Ag ratios higher than ≈ 10, showing how the synergy between both metals is necessary to trigger the enhancement of Ag reduction. The ability of AuAg NSs to generate an electrocatalytic current without the need for any strong acid makes them an extremely promising material for biosensing applications.